Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
Ongoing reductions in feature size and increasing complexity of semiconductor devices impose difficult requirements on optical metrology systems. Optical metrology systems must meet high precision and accuracy requirements for increasingly small metrology targets at high throughput (i.e., short move, acquire, and measure (MAM) times) to remain cost effective. In this context, focusing errors have emerged as a critical, performance limiting issue in the design of optical metrology systems. More specifically, maintaining focus with sufficient accuracy, particularly during high throughput operation (i.e., short MAM times) has become a critical issue for optical metrology systems having high sensitivity to focusing errors.
FIG. 1 depicts an exemplary, prior art metrology system 10 having high sensitivity to focusing errors. Metrology system 10 includes an illumination source 25 that generates a beam of illumination light 14 incidence on a wafer 15. The beam of illumination light 14 passes through illumination pupil 11, illumination field stop 12, and illumination optics 13 as the beam propagates from the illumination source 25 to wafer 15. Beam 14 illuminates a portion of wafer 15 over a measurement spot 16. A beam of collected light 17 is collected from measurement spot 16 by collection optics 18. Collected light 17 passes through collection field stop 19, collection pupil 20, and spectrometer slit 21. The beam of collected light 17 is diffracted by diffraction grating 22 to spatially disperse the beam of collected light according to wavelength. The wavelength dispersed, collected light is incident on the surface of a two dimensional detector (e.g., charge coupled device (CCD) 23. The CCD detector 23 converts the collected light into electrical signals indicative of spectral intensity of the collected light. As depicted in FIG. 1, the collected beam of light 17 includes two distinct wavelengths. Diffraction grating 22 causes a spatial separation between the two different wavelengths of light projected onto the surface of detector 23. In this manner, light collected from measurement spot 16 having a particular wavelength is projected onto detector 23 over spot 24A and light collected from measurement spot 16 having another, different wavelength is projected onto detector 23 over spot 24B.
As depicted in FIG. 1, the Z-axis is oriented normal to the surface of wafer 15. The X and Y axes are coplanar with the surface of wafer 15, and thus perpendicular to the Z-axis. The chief ray 26 of the beam of illumination light 14 and the chief ray 27 of the beam of collected light 17 define a plane of incidence. The X-axis is aligned with the plane of incidence and the Y-axis is orthogonal to the plane of incidence. In this manner, the plane of incidence lies in the XZ plane. The beam of illumination light 14 is incident on the surface of wafer 15 at an angle of incidence, α, with respect to the Z-axis and lies within the plane of incidence.
FIG. 2A depicts a top-view of wafer 15 including a depiction of measurement spot 16 illuminated by the beam of illumination light 14 of FIG. 1. In the embodiment depicted in FIG. 1, the cross-section of the beam of illumination light 14 is circular in shape (e.g., at illumination field stop 12). However, the geometric projection of circular beam 14 onto the surface of wafer 15 results in an measurement spot 16 having an elongated shape aligned with the plane of incidence as depicted in FIG. 2A. For a circular beam of illumination light, the measurement spot 16 projected on the surface of wafer 15 is elliptical in shape. In general, oblique illumination of a surface results in a projected illumination area that is elongated relative to the illumination cross section and the direction of elongation is aligned with the plane of incidence. Moreover, the magnitude of the elongation increases as the angle of incidence increases. More specifically, the beam shape is inversely proportional to the cosine of the angle of incidence in the direction of the plane of incidence. In the absence of diffraction and aberration effects, the projected illumination light remains undistorted in the direction perpendicular to the plane of illumination (e.g., Y-direction).
As depicted in FIG. 1, measurement spot 16 is projected onto the surface of detector 23 in a wavelength dispersive manner. Prior art metrology systems such as metrology system 10 are configured such that the projection of the elongated direction of measurement spot 16 is aligned with the direction of wavelength dispersion on the surface of detector 23. The X′-axis depicted in FIG. 1 is representative of the projection of the elongated direction of measurement spot 16 (i.e., the X-axis) onto detector 23. As depicted in FIG. 1, the X′-axis is aligned with the direction of wavelength dispersion on the surface of detector 23.
FIG. 2B depicts a normal view of the surface of detector 23. As depicted in FIG. 2B, the projection of the elongated direction of measurement spot 16 is aligned with the direction of wavelength dispersion on the surface of detector 23. By way of example, the elongated direction of spots 24A and 24B is aligned with the wavelength dispersion direction. The wavelength dependent images (e.g., spots 24A and 24B) on the surface of detector 23 are integrated in the direction perpendicular to the wavelength dispersion direction to obtain a spectrum, i.e., intensity as a function of wavelength along the wavelength dispersion axis. For a CCD detector, charge is integrated in the direction perpendicular to wavelength dispersion to arrive at the spectrum.
When the measurement spot is imaged onto the detector such that the direction aligned with the plane of incidence on the wafer surface is aligned with the direction of wavelength dispersion on the detector surface, the resulting point spread function (PSF) is strongly wavelength dependent. The resulting PSF is highly peaked because the image intensity varies greatly in the elongated direction for a given wavelength. To properly capture the highly peaked PSD the spectrometer must acquire spectral data at high resolution. This increases measurement time and reduces throughput.
In another example, the resulting PSF for a particular wavelength depends on the angle of incidence when the elongated image, and corresponding elongated intensity distribution, is aligned with the direction of spectral dispersion. The resulting PSF broadens or narrows depending on the angle of incidence.
In another example, the resulting PSF is highly sensitive to focus errors. As the measurement target on wafer moves in and out of focus, the detected image of the measurement spot on the wafer changes size and shifts location. In addition, the location of the measurement spot on the wafer shifts. As illustrated in FIG. 3, when wafer 15 is in focus, the beam of illumination light 14 illuminates the wafer at location A. The beam of collected light 17 is wavelength dispersed and imaged onto detector 23 over spots 24A and 24B as illustrated in FIG. 4. As the wafer 15 is moved upward in the z-direction and is defocused by an amount, ΔZ, that is greater than zero, the beam of illumination light 14 illuminates the wafer at location C. The beam of collected light 17′ is wavelength dispersed and imaged onto detector 23 over spots 24A′ and 24B′. The resulting images are larger as the wafer is moved away from the focal plane of the optical system and the center position of the images shifts in the direction aligned with the wavelength dispersion direction. This shift in the wavelength dispersion direction results in spectral measurement errors as the wavelength to pixel mapping changes. As the wafer 15 is moved downward in the z-direction and is defocused by an amount, ΔZ, that is less than zero, the beam of illumination light 14 illuminates the wafer at location B. The beam of collected light 17″ is wavelength dispersed and imaged onto detector 23 over spots 24A″ and 24B″. Again, the resulting images are larger as the wafer is moved away from the focal plane of the optical system and the center position of the images shifts in the direction aligned with the wavelength dispersion direction.
The measurement spot movement on wafer 15 due to focus error, i.e. ΔZ≠0, results in image movement along the spectrometer dispersive axis as a function of wavelength. Since wavelength calibration is performed in the focal plane, i.e., Z=0, any image movement in the spectrometer dispersive direction induced by focus errors makes the measured spectrum very sensitive to deviations from the wavelength calibration.
In some examples, the emission spectrum of the broadband light source includes one or more characteristic atomic lines, e.g., a Xenon arc lamp. The atomic lines may be used to track and correct focus errors. In prior art metrology systems, focus tracking and correction are essential for achieving measurement accuracy, and tool to tool matching. However, if the broadband light source is a high brightness Laser Driven Light Source (LDLS) the characteristic atomic lines are no longer available for tracking and correction of focus errors. Furthermore, the sensitivity to focus errors becomes exacerbated for large numeric aperture (NA) optical metrology systems.
In summary, sensitivity to focus errors and errors induced by oblique illumination present limitations on the performance of metrology systems, and large NA optical metrology systems, in particular.